Process of smelting zinc containing lead ores



PROGESS OF SMELTING ZINC CONTAINING" LEAD ORES No Drawing.- Application May 15, 1956 Serial No. 584,881

8 Claims. (CI. 75-77) This invention relatesto the process of extracting lead from lead ores containing zinc and is for a method of nited States Patent 8 direct separation of the lead and the zinc from the ore.

Lead is generally obtained from naturally occurring sulphide ore. In cases where the ore or ore concentrate is quite pure, it is smelted-by the well-known roast-re action process. This involves the conversion of the lead sulphide (PbS) with lead compounds containing oxygen, such as lead oxide (PbO) and lead sulphate (PbSO These oxygen bearing compounds may be transiently produced in the smelting process, and it is also well-known to roast or sinter the sulphide ore in the presence of oxygen to produce the oxygen containing compounds mentioned above. The roast reaction may be expressed as follows:

and can be further reduced to the simple formula:

3 PbS+O =Pb+SO It can be carried out simply by blowing an air stream into the heated ore whereby at'approximately 1000 C.

the above reactiontakes place and the molten leadflows down and gathers in a pool coveredby a layer of slag that protects the lead from oxidation.

This procedure is simple; the reaction is exothermic and little or no fuel is required and no addition materials need be used. However it can be used advantageously only with lead sulphide ore of high purity, that is ore which is not only'relatively free of gangue, but which does not contain zinc. Such ore, however, is becoming increasingly unavailable, since zinc in the form ofzinc sulphide is present in appreciable amounts in most ore presently beingproduced.

Zinc sulphide, or zinc oxide produced therefrom in the roast-reaction process combines with the slag, great- 1y increasing is viscosity-and increasing the smelting temperature of the slag to a point where the roast-reaction procedures above described are retarded.

Therefore, the roast-reaction process, can not be used with lead ores containing zinc. Instead, the smelting must be effected in a roast-reduction process in which the lead and zinc sulphides are first completely oxidized and the oxide then reduced in a shaft furnace'with a high grade coke used as a reducing agent. i

The presentinvention is based on the discovery that the roast-reduction of lead sulphide ore concentrates-containing substantial amounts of zinc can be effected by the following procedure:

(1) The charge is prepared from a mixture containing the sulphide ore and more chemically combined oxygen than is required to satisfy reaction number 3 above, and in addition convert the zinc sulphide to zinc oxide. The combined oxygen can be accomplished by sintering ore concentrate on a sintering band in a known manner to partially or completely convert the sulphide to oxide and sulphate.

more basic than zinc oxide and which has a higher dcgree of affinity for the slag than zinc oxide, so that the zinc oxide, instead of reacting with slag, will either be carried off with the furnace gases or reduced by the reducing agent and be carried off asa metallic vapior.

Instead of requiring a shaft furnace with expensive high-grade coke which often times is not available, I may use a reverberating furnace operated withoil, gas'or coal as a smelter. Preferably a short rotating reverberatory furnace with a deep bath is employed. By a short furnace I mean one having adiameter greater than or at least not much less than its length. Such a furnace is used because of its economy and certain other factors that make it highly adaptable for this purpose.

Conventional equipment may be used with myprocess. Roasted and raw ore concentrates may be formed in'the usual way, and a mix prepared that can be sintered' on a Dwight-Lloyd type of downdraft sintering apparatus. A smelting furnace of known construction, preferably as above described, may be used, and finally a dust filter is required for sep'arating'the zinc oxide from the smelter gases. The reducing agent may be carboniferous, as anthracite or coke, or it may be ferrous, as scrap iron, steel or the like. The basic addition for the slag is preferably sodium carbonate (calcined) but other materials of an equivalent character are lime-glass, potash, borax-,- -so,- dium sulphate andlike alkaline materials, or materials for .which the slag impurities have a greater afinity than they do for zincoxide.

In the practice of .the invention, lead ore concentrate containing zinc is r0asted. to. a desired extent. Thisis conveniently accomplished on a downdraft sintering ap} is physically adaptable-for sintering. 'The resultingsinter is crushed or granulated to a convenienfsmall mesh size;

is introduced with Water and the crushed sinter 'into a mixer, the amount of water being suflicient to cause the unroasted material to coat-the coarser roasted'grains. The particles thus produced by agitating or churning the material in the mixer'comprise a core which has already been sinter-roasted surrounded by a layer of non-sinteredr,

ore concentrate.

r The prepared material is then. charged into a short drum smeltingfu'rnace along with the calcined sodiunr carbonate or other basic reagent, and with the reducing agent, which may be broken coal, iron scrap, steel chips or the like, and the charge smelted in the usual way: After the charge has been smelted for about one-third the normal: time, the accumulated lead, generally amounting to about 40-60% of the total lead content, may be drawni off. It will be of a high degree of purity, around 99.9%

pure. Thereafter the smelting operation can be continueduntil the charge has been completely smelted, andflthe:

additional lead drawn off. This metal will be of non mal commercial purity but of a less pure quality than that first removed. A short drum furnace adapts itself well to this procedure.

During the smelting, the gases from the short drum or other smelting furnace are directed through a dust filter or collector. The zinc, instead of being combined in the slag, is recovered by such dust filter or collector as zinc.

oxide.

' As a typical example, 1000 kg. of roasted and sintered lead concentrate was mixed with a like amount of non- Patented .Feb. 23, 1960 I Unroastedore concentrate 'ina finely divided state sintered ore concentrate. The sintered concentrate showed an analysis of 70.6% lead and 4.8% zinc. The nonsintered concentrate showed 70.2% of lead and 5.1% of zinc. From this mixture 100 grams were tested by being subjected to a temperature of 1250 C. in a fire clay crucible. The resulting material had a lead content of 85%, and there was a slag of yellow color, indicating the-presence of lead oxide. This is a quick test to show that more combined oxygen was present than that stoichiometrically required.

Of the mixture 67 kg. were then introduced into a short drum smelting furnace together with 6.7 kg. of calcined soda ash and 4.0 kg. of finely divided anthracite. The soda so added ranges between and 20% of the ore charge, about being indicated as preferable. The reducing agent should be in an amount to assure that the flue gases are non-oxidizing, that is, that all surplus oxygen is reacted. After two hours of firing at a temperature of about 1250 C.',. 45.7 kg. of metal were tapped out of the furnace, and this, was of about 99.5% purity.- There were 6.5 kg. of slag containing about 12,5% of zinc. Also 3.05 kg. of flue dust were collected," and analysis proved this to contain 70% of zinc oxide. The foregoing example shows that 67% of the total zinc was recovered in the flue dust, principally as zinc oxide. At the same time 96% of the total lead was recovered. When operating with concentrates containing higher amounts of zinc than above given, more zinc can be extracted. For example, when an ore concentrate having 8% of zinc and 63.7% of lead was processed in this way, 78% of the total zinc was obtained in the flue dust.

From.the foregoing, it is to be seen that the roastreaction of the lead proceeded normally; the zinc was oxidized and the zinc oxide, which is chemically basic, was prevented from reacting with acid slag and impurities by the use of the more alkaline reagent, such as soda. A surplus of oxygen in combined form over the theoretically'exact amount assures adequate oxygen to effect the process while reducing agent binds the surplus in a manner harmless to the desired result.

While a specific method of preparing the charge has been described, various changes and modifications may be used in the procedure to accomplish the process.

What is claimed is:

1. A process of smelting lead ore containing zinc which comprises initially preparing a charge of roasted and unroasted ore concentrate in a proportion to provide in the mix chemically bound oxygen in excess of the theroetical requirement ofthe roast-reaction process of the lead content plus there being at least suflicient additional chemically bound oxygen to oxidize the zinc to zinc oxide, initially mixing a reducing agent with this charge, then smelting the charge so prepared under conditions conducive to the removal of the zinc as zinc oxide vapor.

2. A process of smelting lead ore containing zinc which comprises preparing a charge of roasted and unroasted ore concentrates in a proportion to provide combined oxygen over and above the amount theoretically required to effect the roast-reaction of the lead and also oxidize the zinc, introducing a reducing agent into the charge before smelting, smelting the charge, and maintaining the slag and impurities sufliciently alkaline that zinc oxide produced in the smelting is carried out with the smelter gases.

3. The methodof smelting lead. ore containing zinc as defined in claim 1, wherein. an alkaline substance more reactive with slag and slag impurities than zinc oxide is introduced into the charge to retard reaction of the zinc oxide with the slag.

4. The method of smelting lead ore containing zinc as defined in claim 1 wherein the charge is prepared by first agitating roasted sintered ore concentrate in granular form with finely divided unroasted ore concentrate and water to coat the sintered granules with the unroasted ore.

5. The process of smelting lead ore containing zinc as defined in claim 2 in which the roasted ore in the form of sintered granules and the non-sin'tered ore concentrate is coated on such granules. I t j 6. A process for smelting lead ore containing zinc which comprises roasting zinc-containing lead sulphide ore'concentrate's to produce mixed lead oxide and lead sulphate, then mixingtherewith additional similar lead ore concentratesin the form of lead sulphide in amoun s such that the combined oxygen is present in the mix in an amount greater than that required for the stoichiometric roast-reaction of sulphur and oxygen, adding to 'said mixture soda and a reducing agent from the group consisting of carbon and iron before smelting, the soda being in an amount suflicient to maintain a slag in the smelting furnace that is essentially basic, smelting the charge so preparedand recovering the lead from the smelting operation in metallic form and the zinc as zinc oxide in flue dust from the smelter.

' 7'. A process for smelting lead ore containing zinc as defined in claim 6 wherein the content of combined oxygen in the furnace charge is adjusted in such manner that substantially thetotal amount of Zinc present in the ore is oxidized to, zinc oxide and the following complete roast-reaction effected:

8. The process of reducing lead sulfide ore concentrate containing zinc which comprises preparing a mixture of ore which is partially roasted to convert the sulfide to lead oxide and lead sulfate while the remainder of th mix is unconverted ore and in which the proportion of combined oxygen is more than sufficient to satisfy the roast-reaction expressed as PbS+O =Pb+SO and con vert the zinc sulfide to zinc oxide, adding to the mix so prepared and before smelting a reducing agent then smelting this mixture to reduce the lead ore to metal and removing the zinc with the smelter gases as zinc oxide.

N :References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION I Patent Nor 2926,0531 February 23, 1960 Werner Schwartz It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected belowc Column 1 line 62, I for "roast-reduction" read roastreaction column 3 line 22 for "125%" read 125% Signed and sealed this 20th day of September 1960a (SEAL) Attest:

KARL Ha AXLINE ROBERT C. WATSON Attesting -Officer I Corm'ussmner of Patents 

1. A PROCESS OF SMELTING LEAD ORE CONTAINING ZINC WHICH COMPRISES INITIALLY PREPARING A CHARGE OF ROASTED AND UNROASTED ORE CONCENTRATE IN A PROPORTION TO PROVIDE IN THE MIX CHEMICALLY BOUND OXYGEN IN EXCESS OF THE THEORETICAL REQUIREMENT OF THE ROAST-REACTION PROCESS OF THE LEAD CONTENT PLUS THERE BEING AT LEAST SUFFICIENT ADDITIONAL CHEMICALLY BOUND OXYGEN TO OXIDIZE THE ZINC TO ZINC OXIDE, INITIALLY MIXING A REDUCING AGENT WITH THIS CHARGE, THEN SMELTING THE CHARGE SO PREPARED UNDER CONDITIONS CONDUCIVE TO THE REMOVAL OF THE ZINC AS ZINC OXIDE VAPOR 